Biological Olfaction - The Electronic Nose Artificial Olfaction Technology - page 71

the 1st cranial nerve) which pass through the perforations in the cribi form plate of
the ethmoid bone and promptly enter the olfactory bulb. These nerve bundles can be
severed as a result of skull fractures or other pathology in this region with a resulting
partial or complete anosmia (loss of sense of smell). Much of the sensation which is
considered to be taste is actually olfactory so patients with anosmia often complain
bitterly about loss of pleasure from eating.
Certain types of chemicals stimulate the receptor cells of smell and taste. The
major functional difference between the two kinds of receptors is that taste
receptors are specialized cells that detect chemicals present in quantity in the
mouth itself, while smell receptors are modified sensory neurons in the nasal
passages that detect vapors coming from distant sources. The smell receptors can
be as much as 3,400 times more sensitive than the taste receptors.
3.3.2 Olfactory Bulb
The olfactory bulbs lie on the ventral aspect of the frontal lobes. The olfactory
bulbs and all other parts of the olfactory pathways are telencephalic derivatives.
Within the olfactory bulbs the olfactory nerves synapse on mitral cells whose
axons project directly to the olfactory cortex. Mitral cells are the principal neurons
in the olfactory bulb. In the olfactory bulbs, the axons of the receptors terminate
and form the olfactory glomeruli. Averages of 26,000 receptor cell axons converge
on each glomerulus. Impulses concerned with olfactory reflexes are passed from
the glomeruli to the rest of the limbic system and the hypothalamus. The olfactory
bulb carries out a great deal of pre-processing and feature extraction of the signals
arriving from the olfactory receptors, analyzing each input pattern and then pro-
ducing specific messages, which it transmits via axons to another part of the
olfactory system, the olfactory cortex. From there, new signals are sent too many
parts of the brain, including an area called the entorhinal cortex, where the signals
are combined with those from other sensory systems. The result is a meaning-
laden perception which is unique to each person, where some scents may produce
a sense of well-being, and others a sense of nausea, and others may be linked to
specific memories or events (Fig. 3.7 ).
3.3.3 Olfactory Cortex
Those portions of the cerebral cortex that receive direct projections from the
olfactory bulb (via mitral cell axons) are collectively referred to as the olfactory
cortex. Note that the olfactory cortex is the one area of cortex that receives direct
sensory input without inter posed thalamic connection. Most of the olfactory
cortex is of a primitive 3-layered type. The olfactory cortex is located on the base
of the frontal lobe and medial aspect of the temporal lobe (Figs. 3.8 , 3.9 ).